Matched-delay local oscillator enabling single-ended vibration localization with a 3-MHz-linewidth DFB laser in ISAC

Single-ended vibration localization based on frequency-shifted optical delay lines avoids the time synchronization required by dual-ended schemes but requires costly narrow-linewidth lasers. To overcome this limitation, we introduce a transmitter-side matched-delay local oscillator (MD-LO) for the sensing unit. The MD-LO matches the sensing-link length and the delay paths between frequency-shifted components, which can tolerate laser phase noise through self-coherent detection. With sensing aided by the MD-LO, vibration detection down to 10 Hz under a 3-MHz-linewidth distributed-feedback (DFB) laser is achieved, and single-ended distributed vibration localization over a 25.2-km weakly coupled four-core fiber is demonstrated. As an illustrative example of integrated sensing and communication (ISAC), the sensing carrier also serves as the local oscillator for self-coherent communication, achieving 3 × 448 Gb/s net transmission over the remaining three cores.